The invention relates to a sensor for detecting an angle of rotation and/or a torque, in particular at axles or shafts, and a method for performing this detection.
From U.S. Pat. No. 5,501,110, a sensor arrangement is already known in which the torque transmitted to a shaft is to be detected. The torque is determined from the torsion, or the offset in rotational angle of the shaft ends and from an elasticity constant, which depends on the material and the geometry of the shaft. Here, two magnets and one Hall sensor facing each magnet are mounted on two disks that rotate with the shaft and that are solidly coupled mechanically to the ends of the shaft.
For instance, to detect the torque acting on a steering wheel shaft of a motor vehicle during the rotation of the steering wheel, very small changes in angle must be measured in both directions of rotation of the steering wheel. In evaluating the field changes in the field originating at the magnets, the known arrangement therefore requires an extremely sensitive and also temperature-stable measurement apparatus.
The torque in the rotating steering wheel spindle is a key variable for many closed- and open-loop control tasks in the motor vehicle. For instance in such vehicle systems as electric motor power steering, adaptive cruise control, and vehicle dynamics control, it is often necessary to detect both variables, that is, the absolute rotational angle of the steering wheel and the torque at the steering spindle.
In principle, the torque can be detected in various ways considered to be known per se. The mechanical strain in the material subjected to torsion is initially the direct measurement variable for the torque, the elongation, or the compression. This mechanical strain can be measured for instance with strain gauges that are applied in the desired direction. In the case of rotating shafts, however, the problem exists of signal transmission; although depending on the application this problem can be solved with a rotation transmitter or with slip rings, nevertheless it is quite expensive and vulnerable to malfunction.
Mechanical strains can also be detected, again in a manner known per se, in contactless fashion using magnetoelastic materials. However, it is extraordinarily difficult to bring about a permanent connection technique between magnetoelastic foils and the torsion shaft. Various methods also exist of detecting a measurement angle by means of the rotational angle offset between different ends of the shaft. This measurement angle can for instance be detected by optical methods or methods based on eddy currents. The optical methods usually have the disadvantage of detecting the angle incrementally and ascertaining the angle and thus the torque from the difference in trigger times, or in other words only dynamically.
It is also known per se from German Patent Disclosure DE 195 06 938 A1 (U.S. Ser. No. 08/894,453) to perform an absolute angle measurement of a rotatable shaft by providing the shaft, whose angular position is to be ascertained with a gear wheel or a ring gear. This gear wheel or ring gear cooperates with two further gear wheels, disposed adjacent one another, whose respective angular positions are ascertained continuously with the aid of a sensor; the rotational angle of the shaft that is to be determined in the final analysis can be ascertained from the two measured angles, using a suitable calculation method.
The sensor arrangement for detecting the rotational angle and/or the torque of rotating mechanical components is advantageously provided, in accordance with the invention, with two angle encoders that are refined according to the invention, which are disposed at different points on the computer, with a torsion bar being formed between them.
In a manner known per se from DE 195 06 938 A1 (U.S. Ser. No. 08/894,453), two bodies rotating in slaved fashion locally at somewhat different speeds of revolution are disposed on the first angle encoder on the circumference of the rotating component; their rotational angles "psgr" and xcex8 are detectable with the sensors, and from their difference in rotational angle, the.absolute rotational angle position xcfx86 of the rotating component can be derived.
According to the invention, the second.angle encoder engages the other end of the torsion bar, and a locally rotating body also engages a sleeve corresponding to the first angle encoder; this rotating body is embodied in terms of its dimensions in accordance with one of the rotating bodies on the first angle encoder. From the measured difference in rotational angle of the rotatable body of the second angle encoder and the identically embodied rotatable body of the first angle encoder, the torque can now be derived at the rotating component in a simple way, since both angle encoders are displaced counter to one another by a torsion caused by the torque.
In an advantageous embodiment according to the invention, the first angle encoder is provided with angle markings or intermeshing sets of teeth on the circumference of the rotating component, in the same way as the rotating bodies. The second angle encoder is likewise provided with angle markings or intermeshing sets of teeth on the circumference of a sleeve and the rotating body. The number of teeth or angle markings on the rotating component and on the sleeve agree. The number m of teeth or angle markings on the rotating body and on one of the rotating bodies on the first angle encoder are also embodied in agreement. Especially preferably, the number m of teeth or angle markings on the two rotating bodies on the first angle encoder differ by one tooth or one angle marking.
In an advantageous method for ascertaining the rotational angle and/or the torque, a first value k1 can be ascertained with the sensor arrangement of the invention for each position of the rotating component, by evaluation of the rotational angles "psgr" and xcex8 of the rotatable bodies on the first angle encoder. A second value k2 can be ascertained by evaluating the rotational angle of the rotatable body on the second angle encoder. The ratio of the values k1 and k2 is monitored, and if there is a change in a ratio and in the angular difference between the respective identical rotatable bodies on the second angle encoder and on the first angle encoder or on the second angle encoder and on the first angle encoder, a torque is detected.
The sensors that detect the angular position of the rotatable bodies can be, magnetoresistive AMR, CMR or GMR sensors, which output a signal that is dependent substantially on the field line direction of the magnets connected to the rotating bodies. These sensors allow use even if installation space is slight and enable a simple electronic evaluation optionally after an analog-to-digital conversion. The evaluation circuit can be a hybrid circuit, which includes the magnetoresistive sensors and a computation circuit for evaluating the measured angles.
In a preferred embodiment, the rotating component is the steering spindle of a motor vehicle, on which spindle the angle encoders are mounted. Once again, because the set of teeth is easy to install on the steering spindle and a torsion bar is embodied with a further set of teeth, a sensor arrangement according to the invention can be produced that requires relatively little installation space. In the line end check in motor vehicle manufacture, the angle differences and the above-described values k1 and k2, as well as their ratio to one another, are detected without torque in straight-ahead driving and are stored in memory. In the ensuing monitoring of the values k1 and k2, these stored values can then be used as a reference.
These and additional characteristics of preferred refinements of the invention are disclosed not only in the claims but also in the specification and the drawings; the individual characteristics can each be realized on their own or together in the form of subsidiary combinations in the embodiment of the invention and in other fields and can represent both advantageous and separately patentable versions for which patent protection is here claimed.